The overall aim of the research proposal is to perform molecular and clinical studies in subjects and affected biologic family members with low levels of high density lipoprotein cholesterol (HDL-C). Whereas an inverse association between HDL-C and coronary artery disease (CAD) is well documented, the genetic basis and potential clinical implications have not been systematically addressed. The specific aims of the proposed research include: 1) Collection and characterization of plasma and DNA from probands with very low HDL-C. Linkage analysis will be performed using highly polymorphic markers within or near HDL- C candidate genes. The hypothesis to be tested is that polymorphic microsatellites segregate with the low HDL-C phenotype. 2) Further genetic characterization of families with evidence of linkage to specific HDL-C candidate genes identified in Specific Aim 1. The hypothesis to be tested is that structural variants in HDL-C candidates are responsible for low HDL-C. 3) Evaluate the physiologic significance of novel genomic variants identified in Specific Aim 2. The hypothesis to be tested is that structural variants will affect expression of the gene product. 4) Examine early atherosclerosis in low HDL-C syndromes. The hypothesis to be tested is that increased carotid intima-medial thickness is prevalent with isolated low HDL-C. Identification of the most extreme forms of this disorder provides a unique opportunity to address these fundamental objectives. Further understanding of the molecular basis of isolated low HDL-C and its potential clinical sequelae (e.g., CAD), may therefore aid in the development of therapeutic strategies to effectively treat this disorder.